Clouds and Precipitation


What are the four major cloud groups and their types?

Clouds are categorized in the following manner:

  1. High clouds are composed almost entirely of ice crystals. The bases of these clouds start at 16,500 feet (5,000 meters) and reach up to 45,000 feet (13,650 meters).

  2. Cirrus clouds (from the Latin for “lock of hair”) are thin, feathery, crystal clouds that appear in patches or narrow bands.

  3. Cirrostratus clouds are thin, white clouds that resemble veils or sheets. These clouds can be striated or fibrous in appearance. Because of the ice content, they are associated with the halos that surround the Sun or Moon.

  4. Cirrocumulus clouds are thin clouds that appear as small, white flakes or cottony patches; they may contain super-cooled water.

  5. Middle clouds are composed primarily of water. The height of the cloud bases range from 6,500 to 23,000 feet (2,000 to 7,000 meters).

  6. Altostratus clouds appear as bluish or grayish veils or layers of clouds that can gradually merge into altocumulus clouds. The Sun may be dimly visible through them, but flat, thick sheets of these clouds can obscure the Sun.

  7. Altocumulus clouds are white or gray and occur in layers or patches of solid clouds with rounded shapes.

  8. Low clouds are composed almost entirely of water and may at times be supercooled; at subfreezing temperatures, snow and ice crystals may be present as well. The bases of these clouds start near Earth’s surface and climb to 6,500 feet (2,000 meters) in the middle latitudes.

  9. Stratus clouds are gray, uniform, sheet-like clouds with a relatively low base, but they can also be patchy, shapeless, low gray clouds. Sometimes thin enough for the Sun to shine through, these clouds bring drizzle and snow.

  10. Stratocumulus clouds are globular, rounded masses that form at the top of the layer.

  11. Nimbostratus clouds are gray or dark, relatively shapeless, massive clouds that contain rain, snow, and ice pellets.

  12. Clouds with vertical development contain super-cooled water above the freezing level and grow to great heights. The cloud bases range from 1,000 feet (300 meters) to 10,000 feet (3,300 meters).

  13. Cumulus clouds are detached, fair-weather clouds with relatively flat bases and dome-shaped tops. These usually do not have extensive vertical development and do not produce precipitation.

  14. Cumulonimbus clouds are unstable, large, vertical clouds with dense boiling tops that bring showers, hail, thunder, and lightning.

  15. The tops of cumulonimbus clouds look like giant cotton balls in the sky.

    Are there other terms for describing clouds?

    Yes, in addition to the major naming conventions for clouds based on altitude and characteristics, there are also a variety of other Latin terms used to describe clouds. These names can be appended to the main names of clouds. For example, a cumulus castellanus is a cumulus cloud with formations on the top that look like castle towers. Below is a full list of other descriptors for clouds.

    Cloud Type Description
    Arcus Arched-or bow-shaped
    Castellanus Turret or tower-like
    Congestus Cauliflower-shaped
    Duplicatus Partly merged, double layered
    Fibratus Fibrous, filament-shaped
    Floccus Wooly, tuft-like
    Fractus Irregularly shaped
    Humilis Flattened, low, and small
    Incus Anvil-shaped
    Intortus Twisted and tangled
    Lacunosus Thin and with holes
    Lenticular Lens-like
    Mammatus Rounded, breast-shaped
    Mediocris Bulging, medium-sized (refers to cumulus clouds)
    Nebulosus Indistinctly shaped
    Opacus Thick
    Pannus Shredded
    Perlucidus Translucent
    Pileus Hood-shaped
    Praecipitatio Rain clouds
    Radiatus Lines of clouds radiating from a central point
    Spissatus Thick, grey cirrus
    Stratiformis Horizontal sheets
    Translucidus Transparent
    Tuba Curved, descending shape
    Uncinus Hooked shapes on the top of cirrus clouds
    Undulatus Wavy
    Uniformis Uniform
    Velum Sail-shaped
    Vertebratus Bone-or ribbed-like

    Mammatus clouds over Tulsa, Oklahoma. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)

    What are nacreous clouds?

    Nacreous clouds are clouds that occur at elevations of 12 to 20 miles (19 to 32 kilometers) high (rarely at lower altitudes) and look like cirrus or altocumulus lenticularis clouds. Often quite beautiful, they are sometimes called “mother-of-pearl clouds” because of irisation: supercooled water droplets causing refraction of sunlight that gives the edges of these clouds multiple colors in a kind of mother of pearl effect. Seen in northern climes, such as Alaska, Scotland, and Scandinavia, these clouds form only a couple hours before sunrise or after sunset.

    What are noctilucent clouds?

    Forming at altitudes of 47 to 56 miles (75 to 90 kilometers), these are the highest clouds you’ll see in our atmosphere. Blown about by upper-atmosphere winds averaging 100 miles (161 kilometers) an hour, these cirrus-like clouds only form during the summer, and only at latitudes of 50 to 75 degrees north and 40 to 60 degrees south. They are usually seen at twilight and have a bluish or silvery color, sometimes flecked with red. It is speculated that noctilucent clouds may form as a result of meteor dust in the upper atmosphere because these clouds are more common when meteor activity increases.

    What is a mare’s tail?

    More technically known as cirrus fibratus clouds, mare’s tails get their name from their appearance. They are long, curved, and fibrous-looking.

    What are the cloudiest U.S. cities?

    In terms of annual average days when they were under overcast skies, the 10 cloudiest cities in the United States are as follows:

    1. Astoria, OR, and Quillayote, WA: both 240 days

    2. Olympia, WA: 229 days

    3. Seattle, WA: 227 days

    4. Portland, OR: 223 days

    5. Kailspell, MT: 213 days

    6. Binghamton, NY: 212 days

    7. Beckley and Elkins, WV: both 211 days

    8. Eugene, OR: 209 days

    What kind of cloud has been mistaken for a UFO?

    Altocumulus lenticularis (commonly called lenticular) clouds are sometimes called “flying saucer clouds” because they bear a strong resemblance to UFOs that have been reported over the years. These clouds, often appearing as one or more lens shapes stacked one on top of another, have been given many other names, too, including cap cloud, banner cloud, rotor cloud, crest cloud, foehn cloud, table cloth, Chinook arch, Bishop wave, and Moazagotl. A peculiar property of these clouds is that they tend to remain stationary, even during winds gusting as much as 150 miles (241 kilometers) per hour. Typically, these clouds are formed near mountainous regions, where air is moving in a “standing wave” pattern. Moist air circulates above the cloud, and water vapor condenses, evaporating as it moves downwind and toward the ground.

    Sometimes, lenticular clouds are mistaken for UFOs because of their unusual disk shapes.

    What is a mackerel sky?

    Mackerel skies are the result of altocumulus or cirrocumulus clouds forming a distinctive pattern that looks like the scales on a mackerel fish’s back.

    What is the Table Cloth cloud?

    The Table Mountain near Cape Town, South Africa, is sometimes covered by a thin sheet of clouds when air is flowing off the top of the mountain in all directions. When this happens, the resulting cloud formation looks like a linen sheet; hence, local people have named it the Table Cloth.

    How much of the Earth is usually covered by clouds?

    At any given time, about one-half of the planet is covered by clouds.

    How do airplanes create clouds?

    When the air conditions are right and it’s sufficiently moist, the exhaust from airplanes often creates condensation trails, known as contrails. Contrails are narrow lines of clouds that usually evaporate rather quickly. Contrails can turn into cirrus clouds if the air is close to being saturated with water vapor.

    What is a contrail?

    The word “contrail” is short for “condensation trail,” and it refers to the water vapor that condenses around the exhaust of a jet aircraft flying at a high altitude. First studied during World War II, when there was a concern that contrails would give away the positions of B29 aircraft, contrails are now of interest to climatologists who worry about their effects on global warming. For instance, studies have shown that where contrails are present there is an increase in the formation of more cirrus clouds. Scientists are also concerned about the other chemicals in jet exhaust that could aversely affect chemical processes within the troposphere and lower atmosphere.


    What is evapotranspiration?

    Evapotranspiration is the combination of water vapor being evaporated from the surface of the Earth (such as from lakes, rivers, or puddles) into the atmosphere, and transpiration, which is the movement of water from plants to the air.

    How does the hydrologic cycle work?

    The movement of water from the atmosphere to the land, rivers, oceans, and plants and then back into the atmosphere is known as the hydrologic cycle. We can pick an arbitrary point in the cycle to begin our examination. Water in the atmosphere forms clouds or fog and falls (precipitates) to the ground. Water then flows into the ground to nourish plants, or into streams that lead to rivers and then to oceans, or it can flow into the groundwater (underground sources of water). Over time, water sitting in puddles, rivers, lakes, and oceans is evaporated into the atmosphere. Water in plants is transpired into the atmosphere, too. The process of water moving into the atmosphere is collectively known as evapotranspiration.

    What is latent heat?

    The concept of latent heat was discovered by Scottish chemist Joseph Black (1728–1799), as well as, independently, by Swiss meteorologist, geologist, and physicist Jean André Deluc (1727–1817). When water condenses, cools, or freezes it loses energy and, thus, gives off heat. Water condensing into vapor releases an amount of energy equal to 600 calories per gram of water; when water freezes, it releases about 80 calories per gram. The heat released provides energy for storms to form.

    What is a “white-out”

    An official definition for “white-out” does not exist. It is a colloquial term that can describe any condition during snowfall that severely restricts visibility. That may mean a blizzard, or snow squall, etc. If you get some sunlight in the mix, that makes the situation even worse—it’s like driving in fog with your headlights on high-beam. The light gets backscattered right into your eyes and you can’t see.

    How fast does rain fall?

    The speed of rainfall varies with drop size, wind speed, and the size of the raindrops. A typical raindrop in still air falls about 7 miles (11 kilometers) per hour. Large raindrops can reach speeds of 16 to 20 miles (26 to 32 kilometers) per hour, while the tiniest drops are slower than a mile (1.6 kilometers) per hour as they drift to Earth. Thus, the larger drops can usually splash onto the ground about three minutes after being generated in a rain cloud that is about 5,000 feet (1,500 meters) above the surface.

    What methods have been used to try to remove hazardous fog?

    Helicopters have been used in the past to try and blow away fog; and in France people have used jet engines at airports to heat up the air. Both of these methods are very impractical, however.

    Are there different categories of rainfall?

    Yes. Rainfall is categorized into three types:

    • Convective rain happens when the Sun warms the air near the ground; as the air then rises, it cools in the higher altitudes and water droplets form, creating a rain shower.

    • Orographic rain is caused when air masses are elevated due to a geological feature such as a mountain. At the same time, land forms create a kind of squeegee effect on moisture as it runs into mountains. The result is the same as with convective rain, because as the air cools in the higher elevation, rain may result on the windward side of the hill or mountain.

    • Cyclonic rain is the result of interacting air masses, which collide and force warm air masses upwards. This type of rain formation often results in strong thunderstorms or hurricanes.

    What is the shape of a raindrop?

    Although a raindrop has been illustrated as being pear-shaped or tear-shaped, highspeed photographs reveal that a large raindrop has a spherical shape with a hole not quite through it (giving it a doughnut-like appearance). Water surface tension pulls the drop into this shape. As a drop larger than 0.08 inch (two millimeters) in diameter falls, it will become distorted. Air pressure flattens its bottom and its sides bulge. If it becomes larger than one-quarter inch (6.4 millimeters) across, it will keep spreading crosswise as it falls and will bulge more at its sides, while at the same time, its middle will thin into a bow-tie shape. Eventually in its path downward, it will divide into two smaller spherical drops.

    How big can a raindrop get?

    The laws of physics restrain raindrops from getting too large before they break up into smaller droplets. Thus, about 0.25 inches (0.635 centimeters) is the largest a drop can get and still have the surface tension of the water hold it together.

    How is rainfall measured?

    Agencies like the National Weather Service use very accurate devices that measure rainfall to the nearest one-hundredth of an inch. The devices, known as rain gauges or tipping-bucket gauges, collect rainwater at a point unaffected by local buildings or trees that may interfere with the rain.

    A NOAA rainwater collector positioned near Mauna Loa, Hawaii, checks for acid rain. (photo Commander John Bortniak, NOAA Corps)

    What is meant by a trace of precipitation?

    When the amount of rainfall is too little to be measured by a standard rain gauge, the precipitation is labeled a “trace.”

    How can I measure how much rain falls where I live?

    Any container with a flat bottom and flat sides can measure rainfall. The width of the top of the container must be the same as at the bottom of the container, but the diameter does not matter. It could be a device purchased for measuring precipitation or something as simple as a coffee can.

    What is the Brückner cycle?

    The Brückner cycle refers to the idea that periods of unusually wet years are then followed by dryer-than-normal years in a cycle that alternates about every 35 years, though it may fluctuate by as little as 20 and as much as 50 years. It is named after German geographer and meteorologist Eduard Brückner (1862–1927). The cycle is also related to colder and warmer years. Brückner, who was also very interested in climate change and glacier advancement and retreats, based his theory on his research into glaciers and tree rings. Because there is so much variation in these cycles, though, climatologists have become much more interested in both the shorter-term and longer-term changes in climate.

    Can some people with arthritis or aching joints predict the rain?

    Many people claim that they can feel a rain storm approaching because they will feel an ache in their knee, a throb in a tooth, or some other pain in their bodies. In a research study conducted by the University of Pennsylvania, scientists learned that, indeed, people with arthritis could sense when humidity levels went up and pressure dropped, both of which are good indications of an approaching storm.

    Where does it rain the most in the United States?

    Mt. Wai’ale’ale, on the island Kauai in Hawaii, receives a whopping average of 460 inches (1,168 centimeters) of rain a year—that’s over 38 feet (about 12 meters) of rain per year!

    What place has the most rainy days every year?

    When it comes to the number of rainy days per year, the winner is again Mount Waialeale on Kauai, Hawaii, with up to 350 rainy days annually.

    Where is the rainiest place on Earth?

    The wettest place in the world, in terms of total rainfall, is Mawsynram, India, which drowns in 468 inches (1,188 centimeters) of rain each year, mostly because of monsoon rains. Nearby is Cherrapunji, India, which receives 460 inches (1,170 centimeters) of rain. Second place, though, goes to Tutunendo, Colombia. Here, the average annual rainfall is 463.4 inches (1,177 centimeters). Unofficially, Lloro, Columbia, endures 523 inches (1,328 centimeters) of rain annually, but no verified measurements have been taken to confirm this claim.

    What are ombrophobia and homichiophobia?

    Ombrophobia is an irrational fear of the rain, while homichiophobia is a fear of fog.

    How much does it rain in the Amazonian rainforest?

    The largest rainforest in the world surrounds the Amazon River basin, most of which lies within the borders of Brazil. Here, the average rainfall is 80 inches (200 centimeters) annually. Interestingly, despite all the rain and thick forest growth, the soil in the Amazon region is quite sterile and not well suited to farming.

    Are stories about fish, frogs, and insects raining down from the skies real?

    While the old saw about “raining cats and dogs” is just an expression, there have been reliable reports of very strange rains during which people are pummeled by frogs, grasshoppers, fish, and other bizarre creatures. For instance, in 1873 it was reported in a Scientific American article that frogs fell from the sky during a storm in Kansas City, Missouri. Both frogs and toads rained down on Minneapolis, Minnesota, during a 1901 storm; more recently, in 1995, a frog storm was reported in Sheffield, England.

    One possible explanation for amphibian rain is waterspouts carrying frogs and toads up into the air, where prevailing winds then dump them onto a distant location. The same sort of theory might apply to reports of raining fish. A couple living in Folsom, California, for instance, reported a fish rain in September 2006, and earlier that year a similar eye-witness account came from Manna, India. Scientists were not incredulous, explaining that waterspouts can kick up winds of 200 miles (320 kilometers) per hour that have been known to lift objects as big as sailboats.

    Other creatures, such as birds and flying insects, have been victims of weird weather, as well. It’s not too much of a mental leap to conceive birds being caught in a strong storm that could injure or disorient them, causing them to fall out of the sky. Insects such as grasshoppers and crickets could just as easily be victims of such storms. In 1988, for example, meteorologists speculated that a swarm of red grasshoppers in Africa was caught up in strong winds and blown all the way to the Caribbean, where they landed in a massive insect rain shower.

    Does it rain in the Arabian Desert?

    Located on the Arabian Peninsula and covering an area of some 900,000 square miles (2,300,000 square kilometers), the Arabian Desert is, indeed, a very dry place, but rain does fall there. Some parts of this desert receive an average annual rainfall of a mere 1.38 inches (35 millimeters). On occasion, flash flooding occurs because of rainstorms. The worst of these occurred in 1995, when a storm and high winds caused flash floods that killed five people near Jiddah.

    Does all precipitation reach the ground?

    No. Rain and other precipitation can often evaporate before reaching the ground, especially when the air is dry (low humidity). In the American Southwest “dry” storms can be quite common, creating lightning and thunder but little precipitation. The danger of evaporating rain is that it can create conditions conducive to downdrafts and microbursts by cooling the air and causing changes in air pressure. Dry storms thus signal a warning to aviators about potential flying hazards.

    Massive dust storms, such as this one near Stratford, Texas, on April 18,1935, were a frightening sight that added to the woes of the Great Depression. (photo by George E. Marsh Album, courtesy NOAA)

    What is virga?

    Virga is a fancy name for rain that dries up before hitting the ground.

    What is the driest place on Earth?

    Probably the driest place on the planet is the Atacama Desert, which is located in Chile near the Pacific coast. The average rainfall here—specifically, in the town of Arica—is about 0.02 inches (0.05 centimeters). Meteorologists believe this never-ending drought is the result of the Humboldt Current, which blocks rain from reaching the Atacama Desert. There are some parts of the Atacama Desert that have not seen a drop of rain in centuries.

    What U.S. drought has been the costliest in terms of financial damages?

    There have been many severe droughts in the United States over the course of history. The most famous one is the Dust Bowl of the 1930s. However, in terms of money, the costliest drought thus far has been the 1988–1989 drought that resulted in an estimated $40 billion hit to the U.S. economy. Over half the country’s population was negatively affected.

    What was the Dust Bowl and what impact did it have on the United States?

    The Dust Bowl drought and dust storms persisted, to varying degrees, from 1933 through 1939, devastating America’s heartland. The worst drought years were in 1934 and 1939, and the worst dust storms occurred in 1935. The Dust Bowl turned once verdant farmlands into wastelands, and huge dust storms swept across Oklahoma, Texas, Kansas, Colorado, New Mexico, and even eastern states. But dry, hot weather was not the only culprit. Farmers at the time used techniques that depleted the soil severely. Most did not rotate their crops or irrigate their lands the way we do so today. The result was that when severe drought hit, crops died and the soil underneath was easily eroded. Strong winds blew the dirt into huge drifts, also sweeping away what little fertile soil was left.

    The impact of the Dust Bowl was not only loss of crops, but also a never-before-seen period of immigration as former farmers abandoned their lands and, in many cases, headed out west to states such as California. Author John Steinbeck captured the plight of these people in his famous 1939 novel, The Grapes of Wrath. Photographers Dorothea Lange and Arthur Rothstein, among others, also recorded these terrible times for posterity in black-and-white photos while working for the Farm Security Administration. Today, the impact of the Dust Bowl can still be seen in America’s central states. There remain many ghost towns that were once prosperous centers of commerce for local farm communities where the soil is still recovering.

    What happened because of a dust storm in 1977?

    One of the worst dust storms to hit the United States since the Dust Bowl occurred in February 1977, when fierce winds blew over plowed fields from Colorado to Texas, kicking up a huge dust cloud. The dust storm dumped three million tons (2.7 metric tons) of soil onto the state of Oklahoma; the cloud continued through Mississippi and Alabama, dramatically raising particulate pollution, and continued across the country and far into the Atlantic Ocean.

    What is drizzle?

    Drizzle is just small droplets of rain measuring, on average, about 0.02 inches (0.05 centimeters) in diameter.

    What place holds the record for the least amount of rain on Earth?

    From October 1903 to January 1918, Arica, Chile, received no measurable rainfall.

    What city in the United States holds the record for a dry spell?

    From October 3, 1912, to November 8, 1914—a period of 767 days—no rain fell on Bagdad, California.

    What does a 40 percent chance of rain really mean?

    When the morning weather report speaks of a 40 percent chance of rain, it means that throughout the area (usually the metropolitan area) there is a 4 in 10 chance that at least 0.001 of an inch of rain (0.0025 centimeters) will fall on any given point in the area.

    Why is it more likely to rain in a city during the week than on the weekend?

    Urban areas have an increased likelihood of precipitation during the work week because intense activity from factories and vehicles produce particles that allow moisture in the atmosphere to form raindrops. These same culprits also produce warm air that rises to create precipitation. A study of the city of Paris found that precipitation increased throughout the week and dropped sharply on Saturday and Sunday.


    What is humidity?

    Humidity refers to the amount—or saturation—of water vapor in the air. Depending on air temperature and pressure, the air can contain differing amounts of humidity before the vapor turns into actual precipitation.

    What is the difference between absolute humidity and relative humidity?

    Absolute humidity is the actual amount of water that is mixed in with the air. It is measured in milligrams per liter. Relative humidity is expressed as a percentage and refers to the actual humidity divided by the maximum water vapor content possible at a given air temperature and pressure. For example: if a liter of air at 98°F (37°C) at one atmosphere pressure contains 44 grams (1.5 ounces) of water vapor, and the actual water content is 11 grams, then the relative humidity would be 25 percent (11/44 × 100 = 25%).

    Can the relative humidity ever be higher than 100 percent?

    No. It was once theorized that when clouds are in a state called “supersaturation” it was possible to have a relative humidity of slightly over 100 percent, but this has since been proven false.

    What is indoor humidity?

    Because we now live in homes with environments controlled by air conditioning, furnaces, and fans, the humidity inside is often very different from what the weather is like outside. For instance, in the winter many homes are very dry inside, which increases static electricity and causes uncomfortably dry skin. Overly humid air within a home can lead to the development of mold and mildew, some of which can be a risk to your health. Also, air that is too dry or too moist can cause structural damage to a house; overly moist wood, too, encourages pests such as termites to chew wooden supports and flooring. A comfortable humidity level for most people is between 30 and 60 percent

    Hygrometers like this one have been used for hundreds of years to measure humidity.

    What is a hygrometer?

    A hygrometer is an instrument that measures the humidity. Italian artist and inventor Leonardo da Vinci (1452–1519) is often credited with constructing the first hygrometer; his design was later improved upon by Francesco Folli (1624–1685) in 1664, and Swiss physicist and geologist Horace Bénédict de Saussure (1740–1799) designed the first mechanical hygrometers. There are basically two types of hygrometers: 1) dry and wet bulb psychrometers, and 2) mechanical hygrometers. The first type uses dry and wet bulb thermometers to compare temperature changes resulting from humidity; the second type uses either organic material (blonde human hairs), which expand or contract based on humidity levels, or semiconductors made of lithium chloride or other substances whose degrees of electrical resistance change according to humidity.

    Who invented the dew point hygrometer?

    The dew point hygrometer (a type of dry and wet bulb psychrometer) was invented in 1820 by John Frederic Daniell (1790–1845). It consisted of two thin glass bulbs joined by a glass tube. One bulb held a thermometer and was filled with ether; the other was empty. As the air in the empty bulb cooled, the ether would condense on the thermometer, indicating the dew point temperature. Variations of Daniell’s device are still used today, including cooled-mirror hygrometers that measure dew point when condensation forms on a mirror.

    Is the relative humidity always 100 percent during times when it is raining or snowing?

    While the formation of rain and snow within a cloud requires humidity to reach a saturation point, by the time the precipitation reaches lower elevations and the ground the rain, hail, or snow can exist within air that has a lower humidity.

    Who invented the device to measure humidity?

    The hygrometer, which measures humidity in the atmosphere, was invented by the French physicist Guillaume Amontons (1663–1705).

    What is humidifier fever?

    If you do not clean your humidifier according to the manufacturer’s recommendations, bacteria and mold may begin to grow on it, which could make you sick. Also, distilled water, not tap water, should be used in a humidifier because dissolved minerals in tap water can cause a fine white powder to blow in your home that can also encourage germs to spread. Though a variety of illnesses, ranging from allergic reactions to the common cold, could develop as a result, if they stem from a poorly maintained humidifier they might all be classified as “humidifier fever.”

    What is the ideal relative humidity that is the most comfortable for people?

    A humidity level between 30 and 60 percent is generally considered comfortable for human beings, while keeping humidity below 50 percent has the added benefit of keeping dust mites under control in homes. Lower humidities tend to lead to dry or cracked skin, itching, and even respiratory problems, and higher humidity causes perspiration to be less effective in controlling body temperature, which makes people feel hotter. In northern climates, where winter dries out the air, humidity can drop below five percent, which is comparable to the humidity levels of a desert.

    What is dew?

    Dew is water vapor that has condensed onto cool surfaces. Usually, this happens overnight as air temperatures cool, and then the dew evaporates as the day progresses.

    What is the dew point?

    The dew point is the temperature at which air is full of moisture and cannot store any more. When the relative humidity is 100 percent, the dew point is either the same as or lower than the air temperature. If a fine film of air contacts a surface and is chilled to below the dew point, then actual dew is formed. This is why dew often forms at night or early morning: as the temperature of the air falls, the amount of water vapor the air can hold also decreases. Excess water vapor then condenses as very small drops on whatever it touches. Fog and clouds develop when sizable volumes of air are cooled to temperatures below the dew point.

    What’s the difference between dew point, humidity, and relative humidity?

    While humidity and dew point are both measures of the amount of water contained in the air, humidity is measured as a percentage of water/air while dew point is indicated by temperature. The dew point indicates the temperature at which the air would become 100 percent saturated. The bigger the difference between the dew point and the current temperature, the less humid the air is. It is therefore the same type of measurement as relative humidity, except that relative humidity is expressed as a percentage and dew point is expressed using temperatures.

    What is the difference between absolute and specific humidity?

    Absolute humidity—given in grams per cubic meter—is the amount of water vapor in a cubic meter of air; specific humidity is the amount of water vapor per gram of air.

    Does dew fall?

    While this is a common expression, dew does not actually fall, a point that was proven in a demonstration conducted by Scottish-American physicist William Charles Wells (1757–1817) in 1814. Wells showed that dew actually formed on surfaces as water vapor condensed.

    Where is the most humid place on Earth?

    In terms of highest dew point temperatures, the extreme is found along the coast of Ethiopia along the Red Sea. Here, the average afternoon dew point in the month of June is a staggering 84°F (28.9°C). Tropical rainforests the world over, of course, are known for extremely high humidity.


    How does snow form?

    Snow forms in clouds in much the same way as water. Water vapor in the clouds collects around nuclei made out of dust or other particles in a cloud. When the temperature within the cloud is cold enough, and the water molecules begin to make contact with one another, they form crystals; and when the crystals become sufficiently heavy, gravity takes over and they fall to the ground.

    The distinctive hexagonal crystal pattern of snowflakes occurs because of the six-fold molecular symmetry of ice. Water is composed of one oxygen bonding with two hydrogen atoms in a kind of V-shape configuration. When the temperature is cold enough and the molecules of water are drawn together, they naturally form into hexagonal rings. This pattern continues as the snowflake grows until it is apparent to the naked eye.

    Does water always freeze at 32°F (0°C)

    The standard that water freezes at 32°F, or 0°C, is only true when the air pressure is exactly one atmosphere and when the water is fresh water and not salt water or some other form of water with impurities in it. Salt water (depending on the percent of salinity) freezes at a lower temperature than fresh water, and when the air pressure is greater than one atmosphere the melting point of fresh water is lowered. However, it takes a lot of pressure to make a difference: to lower the freezing point of water to 30°F (1°C) you must exert a pressure equal to 134 atmospheres!

    Because water droplets in clouds must condense around impurities such as dust, they do not necessarily freeze at 32°F but can remain in a liquid state at temperatures as low as –40°F (–40°C). In other words, the water droplets cannot form ice until they begin to make contact with each other around a nucleus so they can form a crystal lattice. On the other hand, icebergs in the salty oceans are frozen at about the normal freezing point because icebergs are actually formed of fresh water. This happens because ice in the oceans is often formed slowly, allowing the ice crystals to force out impurities such as salt. On the other hand, ice flows—sections of the arctic ice sheet that break away each summer—are composed of sea ice; sea water begins to freeze at 28°F (–2°C) when there is no surface turbulence.

    Another interesting property of ice on frozen lakes or in skating rinks is how friction makes ice skating possible. It was once believed that the pressure of an ice skate blade against the ice surface changed the state of the water from solid to liquid, thus creating a slippery surface on which to glide. More recently, it has been concluded that it is the friction of the blade against the ice that melts it and makes ice skating possible.

    Why don’t lakes and ponds freeze solid in the winter?

    When ice forms in lakes, ponds, and other bodies of water it floats to the surface because liquid water is denser than solid ice. As water cools below 38°F (3.3°C) it becomes less and less dense, and thus more buoyant. This is good news for fish and other plants and animals that live in small bodies of water, since they can live below the layer of ice that forms.

    What is chionophobia?

    Chionophobia is a fear of snow.

    What is the difference between freezing rain, sleet, and hail?

    Freezing rain is rain that falls as a liquid but turns to ice on contact with a freezing object to form a smooth ice coating called glaze. Usually freezing rain only lasts a short time, because it either turns to rain or to snow. Sleet is frozen or partially frozen rain in the form of ice pellets. Sleet forms when rain falls from a warm layer of air, passes through a freezing air layer near the Earth’s surface, and forms hard, clear, tiny ice pellets that can hit the ground so fast that they bounce off with a sharp click. Hail is a larger form of sleet.

    What is the difference between a frost and a hard freeze?

    The National Weather Service issues a hard freeze warning when it predicts that regional temperatures will fall below 27°F (–2.8°C) degrees for four hours or more. Such a freeze is of particular concern to gardeners and farmers because temperatures this low will destroy crops. The good news is that a hard freeze usually spells the end for mosquitoes and other pesky insects for the rest of the year.

    Frosts, on the other hand, do not require such low temperatures. In fact, air temperatures can be several degrees above freezing, while surface temperatures are at or below freezing, causing frost to form on car windows and other surfaces.

    What are glaze and rime?

    Glaze, as one might infer, is the result of freezing rain or drizzle falling onto surfaces and forming a sheet of ice. Rime, on the other hand, is formed by freezing fog or mist during windy conditions. There are two types of rime: hard rime and soft rime. Soft rime has a milky appearance and forms sugar-like crystals shaped into scales, feathers, or needles on the windward side of thin objects such wires, poles, and tree branches. Hard rime is less milky, comb-like in appearance; it is also more dense and less fragile than soft rime. Hard rime can form in windy weather when the temperature is between 18 to 28°F (–2 to –8°C) and humidity is 90 percent or more, while soft rime can occur in similar conditions, but when the temperature is below 18°F (–2°C).

    What is an ice storm?

    Ice storms account for some of the most dangerous winter conditions one can experience. Ice storms occur when freezing rain accumulates on the ground, building up layers of glaze or rime that coat everything, from roads to buildings to telephone lines and vegetation. Naturally, this makes driving extremely hazardous, and many people have lost their lives while traveling in ice storms. Airports will often cancel flights as ice forming on wings can prevent airplane wing flaps from moving, even when workers repeatedly de-ice them. Ice storms can down power lines and even cause older trees to collapse under the sheer weight of frozen water. For example, a tree 50 feet tall with a branch circumference averaging 20 feet can be weighed down by 10,000 pounds (about 4,500 kilograms) of ice as a result of an ice storm.

    Rime ice coats tree branches near Asheville, North Carolina. -(photo by Grant W. Goodge, courtesy NOAA)

    How is hail formed?

    Hail is precipitation consisting of balls of ice. Hailstones usually are made of concentric, or onion-like, layers of ice alternating with partially melted and refrozen snow, structured around a tinycentral core. It is formed in cumulonimbus or thunderclouds when freezing water and ice cling to small particles in the air, such as dust. The winds in the clouds blow the particles through zones of different temperatures, causing them to accumulate additional layers of ice and melting snow and to increase in size.

    Texas is well known for its large hailstones, like this one that fell during a June 8,1995, storm. (NOAA Photo Library, NOAA Central Library; OAR/ERL/National Severe Storms Laboratory)

    Are hailstones always round?

    Usually, hailstones are round or lumpy-round little ice balls. Sometimes, however, they can be oblong or have protruding spikes.

    How large can hailstones become?

    The average hailstone is about one-quarter inch (0.64 centimeter) in diameter. However, hailstones weighing up to 7.5 pounds (3.4 kilograms) are reported to have fallen in Hyderabad state in India in 1939, although scientists think these huge hailstones may be several stones that partly melted and stuck together. On April 14, 1986, hailstones weighing 2.5 pounds (one kilogram) were reported to have fallen in the Gopalgang district of Bangladesh; there was also a report of a hailstone weighing 4.5 pounds (2.04 kilograms) in Germany.

    The current claim to fame for largest hailstone found in the United States was one that was measured to have a circumference of 18.75 inches (47.625 centimeters) that fell in Aurora, Nebraska, in June 2003. Before that, the record was held by a 17.5-inch (44.45-centimeter) stone found in Coffeyville, Kansas, in September 1970. When it comes to weight, however, the Coffeyville stone was 1.67 pounds (0.76 kilograms) while the Aurora hailstone was a fluffier 1.3 pounds (0.59 kilograms).

    What are the different size categories for hailstones?

    In the United States, the following terms are used to describe the size of hailstones in weather reports. The actual sizes of the hailstones often don’t match up to their supposed similes.

    Hailstone Description Diameter (inches/centimeters)
    Pea   0.25 / 0.65
    Marble   0.50 / 1.25
    Penny, dime, large marble   0.75 / 1.90
    Nickel, mothball   0.88 / 2.25
    Quarter   1.00 / 2.50
    Half dollar   1.25 / 3.20
    Walnut   1.50 / 3.80
    Golf ball   1.75 / 4.45
    Hen egg   2.00 / 5.00
    Tennis ball   2.50 / 6.35
    Baseball   2.75 / 7.00
    Tea cup   3.00 / 7.60
    Grapefruit 4.00 / 10.25
    Softball 4.50 / 11.40

    Is there such a thing as a megacryometeor?

    Yes. Megacryometeors are not hailstones because they don’t form in clouds the way hailstones do. They are giant ice stones that do not require thunderstorms in order to form. Megacryometeors range in size from about a third of a pound (half a kilogram) to a monstrous one found in Brazil that weighed 137 pounds (62 kilograms). But how do they form? One argument is that they come from airplanes, but not from toilet drainage because this would mean contaminants and cleaners would be found in these large ice stones. However, ice might still form and break off of airplanes while in flight, and since it could take as much as three minutes for such a piece of ice to reach the ground, such megacryometeors might seem to come from out of the blue because the jet plane is long gone by then. A growing number of meteorologists, however, do not buy into the airplane scenario. They speculate that cooling and water vapor conditions within the tropopause that are still little understood may explain the formation of megacryometeors.

    What are the “hail belts”

    Hail belts are regions that are ideal for hail storm formation. They can be found downwind of mountain ranges, usually in mid-latitude areas. Among the hail belt 1 regions are the Central Plains in the United States and Canada, Central Europe and parts of the Ukraine, southern China, Argentina, and parts of Central and South Africa and southeastern Australia.

    Was the huge ice stone found in Hartford, Connecticut, really a hailstone?

    No. On April 30, 1985, a 13-year-old-boy found a block of ice that was 1,500 pounds (680 kilograms), which had apparently fallen from the sky and onto his backyard lawn. No one ever figured out where it came from (sometimes, though, chunks of ice can fall from passing airplanes), but it was definitely not a natural hailstone.

    What are some notable record hail accumulations?

    In 1968, a storm in Illinois dumped the equivalent of 82 million cubic feet (2.32 million cubic meters) of ice over a 620,000 acre area. Then, in 1980, a hail storm in Orient, Iowa, left hail drifts that were six feet (1.8 meters) deep.

    What U.S. cities tend to get the most hail?

    The following U.S. cities are on the Top 10 list for receiving the most hail annually.

    • Cheyenne, WY

    • Tulsa, OK

    • Amarillo, TX

    • Oklahoma City, OK

    • Wichita, KS

    • Dallas/Ft. Worth, TX

    • Arlington, TX

    • Denver, CO

    • Colorado Springs, CO

    • Shreveport, LA

    Do hailstones have rings?

    Yes, but unlike with trees, the rings do not indicate the age of the hailstone. Italian physicist Alessandro Volta (1745–1827) did a study in 1806 to analyze the bands. He concluded that hailstones, before they fall to the ground, may change elevation a number of times due to up-and downdrafts. As the temperature and moisture levels change, layers of ice accumulate on an “embryo” (nucleus) at different rates, thus forming the layers.

    Have hailstones ever proven to be deadly?

    Strong hails can be heavy and plummet at speeds that have been known to break windows, dent cars, damage roofs, destroy crops (to the tune of about a billion dollars annually in the United States alone), and, yes, even kill. In July 1953, for instance, 30,000 ducks were found dead after a hail storm in Alberta, Canada. In Montana, a July 1978 hail storm killed about 200 sheep.

    In the United States, not many people have been killed as a result of hail. The last reported death was on July 30, 1979. A hail storm in Fort Collins, Colorado, killed an infant and injured about 70 other people. Storms in other, poorer regions of the world are more common, however, because poorly built shelters collapse, killing those inside. For example, in Sichuan Province, China, a fierce hail storm injured 9,000 people and killed 100 on March 22, 1986. A month later, that same year, 92 people died in Gopolganj, Bangladesh, with some hailstones reported to weigh over two pounds (three kilograms).

    What is clear ice?

    Clear ice, as the name suggests, is clear, amorphous ice forming on surfaces as a result of water drops falling and freezing when the temperature is between 27 and 32°F (-3 to 0°C).

    What is graupel?

    Graupel—also known as snow pellets—is a funny word that means soft hail. It is created when supercooled water causes rime to form around a snowflake nucleus. Graupel is heavier and more granular than regular snow. Because of this, large amounts of graupel forming on a hill or mountainside can create dangerous conditions suitable for avalanches.

    Frost coats a window with beautiful crystalline patterns.

    When does frost form?

    A frost is a crystalline deposit of small thin ice crystals formed on objects that are at freezing or below freezing temperatures. This phenomenon occurs when atmospheric water vapor condenses directly into ice without first becoming a liquid; this process is called sublimation. Usually frost appears on clear, calm nights, especially during early autumn when the air above the Earth is quite moist. Permafrost is ground permanently frozen that never thaws out completely.

    Besides rime frost, what other types of frost are there?

    Different weather conditions can cause frost to form in various ways, sometimes with spectacularly beautiful results. The types of frost include:

    Advection (wind) frost is frost that forms on the edges of plants and other objects. Advection frost is formed on the upwind side of objects during very cold winds.

    Fern (window) frost gets its name from the fernlike patterns it forms on windows, especially windows that are not well insulated. Flaws in the glass’s surface provide the nucleus needed for water vapors to form crystals, which then radiate outwards in intricate patterns.

    Frost flowers are the result of a rare interaction between plants and the weather. When water inside a plant stem cracks or splits due to the cold, the water can escape and then freeze into flower-like shapes. Because they are so fragile, frost flowers usually break apart or melt within hours of forming.

    Hoar (radiation) frost is formed on clear nights when surface objects are colder than the surrounding air. It appears as white, loosely organized crystals. Hoar frost may appear similar to rime, but unlike rime it is formed without the presence of mist or fog.

    What is the difference between snow and hail?

    Snow is water vapor that freezes in clouds before falling to the Earth. Hail is water droplets (raindrops) that have turned to ice in clouds.

    How much water is contained in snow?

    Depending on conditions—and as anyone who has had to shovel snow can attest to—snow can range from light and fluffy to heavy, dense, and slushy. As a general rule of thumb, however, every 10 inches of snowfall that accumulates on the ground would equal about an inch of rain if it all melted.

    Is it true that no two snowflakes are exactly the same shape?

    Some snowflakes may have strikingly similar shapes, but these twins are probably not molecularly identical. In 1986, cloud physicist Nancy Knight believed she found a uniquely cloned pair of crystals on an oil coated slide that had been hanging from an airplane. This pair may have been the result of breaking off from a star crystal, or were attached side by side, thereby experiencing the same weather conditions simultaneously. Unfortunately the smaller aspects of each of the snow crystals could not be studied because the photograph was unable to capture possible molecular differences. So, even if the human eye may see twin flakes, on a minuscule level these flakes are different.

    When were huge snowflakes observed?

    On January 28, 1887, in Ft. Keough, Montana, there was a snowfall that included flakes measuring a spectacular 15 inches (38.1 centimeters) across! Of course, these flakes were not individual crystals, but rather clumps of crystals sticking together to form large flakes. Not long after these stunning flakes were seen, Shirenewton, England, experienced a storm in 1888 where 3.75-inch (9.5-centimeter) snowflakes were seen.

    Can the form of snowflake crystals be predicted?

    Snowflake crystals come in several forms, including needle-shaped, platelike, capped columns, and feathery dendrites. Temperature and humidity levels determine which type of shape is formed, so, yes, if these conditions are known, the type of snowflake formed could be predicted. In natural conditions, this would of course be impractical, but laboratory conditions could be established to form particular types of snow crystals, if desired.

    How unhealthy is snow shoveling?

    Heart attack rates increase sharply during the winter months in northern climates because people who are older or are not very healthy get too much exercise shoveling snow. Because more men than women tend to shovel snow, about three fourths of winter fatalities after snow storms are men. Fifty percent of these men, too, are over 60 years of age. You should always ask a doctor if you are healthy enough to shovel snow

    Where was the most snowfall ever recorded?

    Washington State’s Mt. Baker recorded the most snowfall in a single season: 1,140 inches (2,896 centimeters).

    What is diamond dust?

    Diamond dust, also known as “ice prisms,” are tiny ice crystals that can form in the air on extremely cold days if the air contains enough moisture. The effect can be quite beautiful, as sparkling, barely visible crystals appear in mid-air on sunny days, catching the sunlight and, indeed, appearing as if they are tiny diamond chips wafting in the breeze.

    Who created the first artificial snowflake?

    Japanese physicist Ukichiro Nakaya (1900–1962) created the first artificial snowflakes at Hokkaido University in 1936. Nakaya, who was inspired by the photographs of Wilson A. Bentley (1865–1931), also devised a rather poetic snowflake classification system, which he described in his 1954 book, Snow Crystals: Natural and Artificial.

    Wilson A. Bentley was famous for taking highly detailed photographs of snowflakes, such as these images taken in 1902. (NOAA)

    Is there a classification system for snowflakes?

    Humankind has an affinity for classifying just about anything, and that includes snowflakes. In 1951, the International Commission on Snow and Ice (yes, there was such a commission!) created a system for putting a name on each type of snowflake—a daunting task when one considers that no two flakes are alike.

    What are the types of snowflakes as identified in the International Classification System?

    The types of snowflakes have been described, officially, as follows.

    • Stellar Plates, as the name indicates, are starlike flakes that are flat, distinctly hexagonal, with six broad arms.

    • Sectored Plates are similar to stellar plates, but also have prominent ridges pointing to each of the six facets in the plate.

    • Double Plates occur when two stellar plates are connected by a cap. Usually, one plate is much larger than the other.

    • Split Plates and Stars happen when parts of two separate plates merge to form one plate that, if not closely inspected, looks like a single six-armed plate. For example, a partial plate containing two arms could merge with one that has four arms left, leaving a six-armed plate that appears like a complete single plate.

    • Simple Prisma—tiny, six-armed, flat snowflakes that are hard to distinguish with the naked eye, but are a very common form.

    • Stellar Dendrites have treelike arms (dendritic means “treelike”) with multiple branches extending from each of the six arms.

    • Fernlike Stellar Dendrites are stellar dendrites with more frilly, fernlike branches.

    • Radiating Dendrites (Spacial Dendrites) are dendrites that have arms extending not just in two dimensions, but in three.

    • Capped Columns look like columns that have six flat sides (imagine two hexagons that are joined together).

    • Hollow Columns are similar to capped columns, except the ends of the columns are hollow or divoted.

    • 12-sided Snowflakes When two six-sided plates join together at a 30-degree angle, they form what appears to be a 12-armed plate or capped column.

    • Needles look just like the name: thin, long ice crystals. They usually form when the temperature is about 23°F (–5°C).

    • Triangular Crystals often form at temperatures of about 28°F (–2°C) and are rather like deformed stellar flakes where half the arms are not fully formed, creating a triangle shape as a result.

    • Bullet Rosettes happen when several columns melt and freeze together, looking like several crystal bullets merged at the heads at odd angles to each other.

    • Rimed Crystals occur when additional water droplets freeze onto already formed snowflakes, giving them a fuzzy appearance.

    • Irregular Crystals are snowflakes that are a rather disorganized mess of multiple snowflakes that have broken up and melted together.

    Who was the “Snowflake Man”

    American photographer and farmer Wilson A. Bentley (1865–1931) was nicknamed the “Snowflake Man,” or just “Snowflake” Bentley, because he photographed images of over 2,400 snowflakes. His stunning photo collection capturing the natural beauty of snowflakes was published in 1931’s Snow Crystals.

    What are some record snowfalls over a 24-hour period in the United States?

    There have been a number of impressive snowfall records in the United States since the 1990s, including the following:

    Record One-Day Snowfalls

    Place Date Snow (inches/cm)
    Valdez, AK January 15, 1990 47.5/120.6
    Deadwood, SD November 24, 2008 43.6/110.7
    Buffalo, NY December 9-10, 1995   37.9/96.3
    New York, NY February 11-12, 2006   26.9/68.3
    Glasgow, MT October 12, 2008   12.8/32.5

    What is therecord for the greatest snowfall in the United States?

    The record for the most snow in a single storm is 189 inches (480 centimeters) at Mount Shasta Ski Bowl in California from February 13–19, 1959. For the most snow in a 24-hour day, the record goes to Silver Lake, Colorado, on April 14–15, 1921, with 76 inches (193 centimeters) of snow. The year record goes to Paradise Ranger Station, Mount Rainier, in Washington with 1,224.5 inches (3,110 centimeters) from February 19, 1971, to February 18, 1972; Paradise also has the highest average annual snowfall with 680 inches (1,727 centimeters). In March 1911, Tamarack, California, had the deepest snow accumulation—over 37.5 feet (11.4 meters).

    Snow fences like this one are used to prevent snow drifts from encroaching on buildings.

    Has it ever snowed in Arizona?

    Higher elevations in the state actually receive quite a bit of snow. Flagstaff, for instance, is situated at an elevation of about 7,000 feet (2,133 meters), and experiences very cold temperatures in the winter (in January 22, 1937, the temperature dipped to –30°F [–34.4°C]). Though the climate of Flagstaff is officially “semi-arid,” the city receives 100 inches (254 centimeters) of snow annually. Even in lower elevations, however, snow is not unknown in Arizona. Tucson received an extraordinary 6.4 inches (16.5 centimeters) of snow on November 16, 1957. This, of course, was not typical for the otherwise very hot city.

    What are some other unexpected places where it has snowed in the United States?

    In Louisiana, two feet (61 centimeters) of snow fell on the town of Rayne on February 14, 1895. Another old record is held by Savannah, Georgia, where 18 inches (46 centimeters) of the white stuff fell on January 10, 1800. More recently, residents of New Orleans were surprised to see snow accumulations on their front lawns on December 11, 2008.

    What is a heavy snow warning?

    A heavy snow warning is issued when the National Weather Service expects an accumulation of four inches (10 centimeters) or more within a 12-hour period. Heavy snow warnings differ from blizzard warnings in that they do not depend on strong winds for an advisory to be issued.

    What is the purpose of a snow fence?

    Snow fences are usually installed in places where there is not much in the way of vegetation or buildings to keep winds from blowing and drifting snow. The fences create wind turbulence that prevents drifting downwind of the fence.

    What is a snow roller?

    People aren’t the only ones who enjoy building snowmen. Sometimes, nature gets into the game as well. In windy, wintry conditions, breezes have been known to start small collections of snowflakes rolling. As they roll, snow accumulates, and the snowball gets bigger and bigger. Such snow rollers have been known to grow to diameters of several feet.

    What are the 10 snowiest cities in the United States?

    The annual average snow precipitation for the 10 U.S. cities that have the most snow is listed below.

    Top 10 Snowiest U.S. Cities*

    City Average Annual Snowfall (inches/cm)
    Truckee, CA 203.4/516.6
    Steamboat Springs, CO 173.3/440.2
    Oswego, NY 153.3/389.4
    Sault Ste. Marie, MI 131.2/333.2
    Syracuse, NY 120.2/305.3
    Marquette, MI 118.2/300.2**
    Meadville, PA 111.2/282.4
    Flagstaff, AZ 111.1/282.2
    Watertown, NY 110.8/281.4
    Muskegon, MI 105.9/268.9
    * Cities included have a population of 10,000 people or more; statistics are averages from 1971 to 2000.
    ** The Marquette airport gets 179.8 inches (456.7 centimeters) on average.

    Is it ever too cold to snow?

    No matter how cold the air gets, it still contains some moisture, and this can fall out of the air in the form of very small snow crystals. Very cold air is associated with no snow because it is usually very dry and these invasions of air from northerly latitudes are associated with clearing conditions behind cold fronts. Heavy snowfalls are associated with relatively mild air in advance of a warm front or on the back side of a strong low pressure system. The fact that snow piles up, year after year, in Arctic regions illustrates that it is never too cold to snow.

    Can you use snow to keep you warm?

    Snow, just like wood or stone or soil, can be used as an insulator. If there is sufficient snow on the ground, one can dig out a small cave or igloo-like structure, crawl inside of it, and wait for one’s body heat to warm the inside. To work more efficiently, it is best to be six or more feet (two meters) below the surface, and keep the opening of your snow cave downwind. Such makeshift structures can warm up to 60°F (15°C) or more, and can be quite cozy. They are a good way to avoid exposure if one has become lost or stranded outdoors in the winter.

    How warm can the weather be and yet have snowfall?

    It is possible for the temperature on the ground to be in the 40s Fahrenheit (4 to 9°C) and still snow. This happens when snow forms at colder temperatures in clouds and does not melt before reaching the ground. In one case, a temperature of 47°F (8°C) was measured at New York City’s LaGuardia Airport when snow was also seen.

    What is the snow line?

    The term snow line can refer to two different things: 1) the elevation on a hill or mountainside above which snow has fallen and below which the precipitation turns to rain; or 2) the latitude north of which is covered by snow.

    What are you seeing when you “see your breath”

    It can be fun for kids to breathe out on a cold day and pretend that they are perhaps dragons, but it is obviously not smoke coming out of their mouths: it is water vapor. What is happening is that the moisture in one’s breath (humidity) is turning to fog as it leaves the warm confines of the mouth and hits the chilly air. While one or two people breathing out on cold days will not affect the weather, it has been observed that large herds of animals huddling together on a winter’s day can actually produce a small fog bank.

    What places experience the most annual precipitation on Earth?

    Highest Annual Precipitation

    Place Precipitation (inches/cm) Years on Record
    Lloro, Columbia 523.6/1,330.9* 29
    Mawsynram, India 467.4/1,187.2 38
    Mt. Waialeale, Kauai, HI 460/11,68.4 30
    Debundscha, Cameroon 405/1,028.7 32
    Quibdo, Columbia    354/899.2* 16
    Bellenden Ker, Queensland, Australia    340/863.6 9
    Henderson Lake, British Columbia, Canada    256/650.2 14
    Crkvica, Bosnia-Herzegovina    183/464.8 22
    * Estimated.

    What is themost rainfall ever recorded within a one-day period?

    Tropical Storm Claudette brought a record U.S. rainfall of 43 inches in 24 hours in and around Alvin, Texas, in July of 1979.

    The coastline near Arica, Chile, known as the driest place on Earth.

    What places have the lowest annual precipitation on Earth?

    Lowest Annual Precipitation

    Place Precipitation (inches/cm) Years on Record
    Arica, Chile    0.03/0.076 59
    Place Precipitation (inches/cm) Years on Record
    Amundsen-Scott Station, Antarctica 0.08/0.2 10
    Wadi Halfa, Sudan 0.1/0.025 39
    Batagues, Mexico 1.2/3.05 14
    Zaranj, Afghanistan 1.36/3.45 N/A
    Aden, Yemen 1.8/4.57 50
    Mulka, South Australia 4.05/10.29 42
    Astrakhan, Russia 6.4/16.26 25
    Puako, HI 8.93/22.68 13


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